An investigation of binary star formation and evolution with surveys of three open clusters.

摘要

Through high angular resolution multiplicity surveys of 410 members of 3 young open clusters---the Hyades, alpha Persei, and Praesepe---this thesis was designed to test binary star formation and evolution scenarios as well as the effects of companion stars. The 2.2 mum speckle imaging on the IRTF 3-m, the Hale 5-m, and the Keck 10-m formed most of the data set, but were complemented with direct imaging with NICMOS aboard the Hubble Space Telescope (HST). For systems with magnitude differences of DeltaK ≤ 4mag, 62 systems have been resolved with separations that encompass 0″ .027 to ∼5.″ The measured companion star fraction (CSF5-50AU) for the Hyades (0.16 +/- 0.03) is intermediate between the younger T Tauri stars and older G-dwarfs, but the CSF25-561AU of alpha Persei (0.10 +/- 0.03) and Praesepe (0.10 +/- 0.03) are both consistent with the G-dwarf value, suggesting that there is not a systematic decline in multiplicity with age. The higher CSF of the Hyades is explained by the overall separation distribution of cluster binaries. Merging the current cluster surveys with previous spectroscopic work results in a cluster binary distribution that peaks at 5 AU, a significantly smaller value than the peaks of both the G-dwarf (∼30 AU) and T Tauri (∼60 AU) distributions. If the field G-dwarf distribution represents a superposition of populations which contributed to the field, then the data implies that ∼33% of field binaries formed in T associations and the remaining ∼67% formed in the dense progenitors of open clusters. An exploration of the binary star properties reveals a number of observational trends that are inconsistent with several models of binary star formation involving capture, fragmentation, and accretion. Among the B to K stars observed, the CSF increases with decreasing target mass, while the mass ratio distribution of lower mass stars shows a deficit of low mass ratio binaries compared to higher mass stars. Finally, for the youngest cluster, the frequency of companions is not correlated with stellar rotation, while the rotational velocities for binaries with separations less than 55 AU are significantly higher than that of wider systems. This suggests that close companions may critically affect the rotational evolution of young stars.